1. Field of the Invention
The present invention relates to a method and apparatus for setting the color balance of a color display device such as a visual display unit for a computer system.
2. Description of the Related Art
An image generated by a color visual display device is defined by red, green and blue image components. These image components are generated by red, green and blue video signals. The color balance, which is sometimes referred to as the color point, of the display device is a measure of the relative intensities of these components when the video signals are arranged to produce a standard image in the form of a white block. The color point is generally specified as the coordinates corresponding to a white block on a chromaticity reference chart such as the well known CIE 1931 (x,y) Chromaticity Diagram or the like. In a color display device, the color point is determined by the relative gains of the red, green, and blue video channels of the device.
For example, in a cathode ray tube color display device, the color point is determined by phosphor coatings of the cathode ray tube, and by the relative gains of video amplifiers connected to the red, green, and blue electron guns. The gains of the video channels are dependent on electrical circuit parameters. These parameters may vary between different display devices manufactured on the same production line. There may therefore be a corresponding variation in color point between the devices. To compensate for such variation, the video channels are usually provided with adjustable gain controls. The gain controls permit color point adjustment when the display device is tested at the end of the production line. In a typical high volume manufacturing environment, this is performed in a manually operated test cell.
The test cell conventionally comprises a video signal generator and a television color analyzer (TVCA). The video signal generator is configured to generate red, green, and blue video signals for producing a standard white block image on a display device under test. The TVCA measures the chromaticity of the white block produced on the display device and produces outputs in the form of the coordinates (x,y) of the color point of the image on the chromaticity reference chart. An operator adjusts the gains of the video channels until the desired color point is obtained. Because each video channel gain affects the color point of the image, it requires skill to judge which gain control must be adjusted to correct the color point and obtaining the desired color point may involve several iterations of adjustment. This is time consuming and therefore impedes productivity in a high volume manufacturing environment.
U.S. Pat. No. 4,123,775 describes an apparatus and method for setting the color point of a color display device to a desired value. The apparatus comprises three photodetectors which are respectively sensitive to red, green and blue wavelengths of light. The photodetectors are connected to analog processing circuitry for driving an oscilloscope. In use, the photodetectors are directed towards an image displayed on a display device under test. The processing circuitry processes electrical signals from each of the photodetectors to generate a pair of color difference signals. The difference signals are applied to the horizontal and vertical timebases of an oscilloscope. The video channels of the display device are then adjusted until the trace on the oscilloscope reaches a "null" point indicating the desired color point has been obtained.
Because this apparatus operates by comparing two color difference signals, several iterations of adjustment may be required before the desired color point is obtained. The likelihood of this is increased where the display device under test comprises a CRT, in which there is usually an interaction between the red, green and blue electron guns during color point adjustment. Furthermore, because component values in the analog processing circuitry may vary, the apparatus requires periodic recalibration. Recalibration of the processing circuitry may also be necessary each time the required the desired color point is changed.
It will therefore be appreciated that this apparatus is unsuitable for a high volume manufacturing environment.